#16 Unsafe ALTER TABLE DISTRIBUTE operation

In general case ALTER TABLE DISTRIBUTE basically copies table data to a store, then truncates table, then copies data back according to new distribution.
If last operation fails, all table data are lost.

Proposed approach: internally create new table, copy over data, drop old table and rename new to the old name.
Or maybe a low-level approach: copy data to the new pages (files) and wire them up to the existing pg_class instance.